The present invention relates to a light source and an illumination system for use with an extreme ultraviolet (“EUV”) exposure apparatus that transfers a fine pattern in semiconductor manufacturing.
In manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology, a reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (such as i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the photolithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical apparatus using the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light (referred to as “EUV exposure apparatus”) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The EUV light source uses, for example, a laser plasma light source. It irradiates a highly intensified pulse laser beam to a target material put in a vacuum chamber to generate high-temperature plasma for use as the EUV light with a wavelength of about 13 nm emitted from this. The target material may use Xe gas, droplets, and clusters, and a metallic thin film, such as copper, tin, aluminum, etc., and is supplied to the vacuum chamber by gas jetting means and other means.
The laser plasma as one mode of the EUV light source irradiates the high-strength pulse laser light onto the target material and generates not only the EUV light from the target material, but also flying particles called debris, which causes pollution, damages and lowered reflectance of an optical element. Accordingly, a method have been conventionally proposed which mitigates influence of debris by providing a foil trap made of a porous material around the target material and circulating inert gas, such as He gas, as buffer gas.
Since He gas as well as Xe gas as the target material is essential to a light emitting section of the target material, the pressure in a vacuum chamber becomes about 10 Pa although a vacuum pump exhausts the chamber. The atmosphere of a stage subsequent to the light emitting section should be maintained as clean as possible, preferably with the degree of vacuum of about 10−7 Pa, for intended performance such as reflectance of the optical element, since the EUV light has low transmittance to the air and contaminates an optical element when reacted with a residual gas component (such as high molecule organic gas).
Differential pumping system have already been proposed which use a thin film window provided between a light emitting section and an optical element in a stage subsequent to the light emitting section (as seen in Japanese Patent Applications Publications Nos. 5-82417, and 2-156200). Several proposals of exposure dose control over a pulsed light source may be seen in U.S. Pat. No. 5,305,364.
It is difficult to manufacture and handle a self-supported filter material that has high transmittance and is applicable to a wavelength range of the EUV light. A differential pumping method is conceivable, as shown in FIG. 8, which uses a channel or orifice 3900 for differential pumping at a connection between a light source chamber 3110 that accommodates a light emitting section and an illumination system chamber 3120 that stores an optical element 3500. Here, FIG. 8 is a schematic structure of an EUV light source 3000 that uses a laser plasma light source.
The differential pumping using the orifice 15 generates a pressure difference of about 10−2 Pa between the light source chamber 3110 and an illumination system chamber 3120. When it is considered that the light source chamber 3110 has the pressure of about 10 Pa as discussed, the pressure in the illumination system 3120 becomes about 10−1 Pa, which is insufficient to maintain the performance such as the reflectance of the optical element 3500.
In order to obtain a desired pressure difference between the light source chamber 3110 and the illumination system 3120, it is conceivable to elongate the channel 3900 that connects the light source chamber 3110 and the illumination system chamber 3200. On the other hand, for enhanced use efficiency of the EUV light 3400, a spheroid condenser mirror 3600 should capture the EUV light generated from the target material as much as possible, for example, at about Π steradian. However, as the capture angle becomes large, it becomes difficult to elongate the channel 3900 and to obtain a desired pressure difference.
A demand to maintain the pressure in the illumination system chamber to be the degree of vacuum of about 10−7 Pa is common to a discharge method that generates the EUV light by circulating Xe gas, etc. in an electrode for discharging and generating plasma, as well as the laser plasma method.
Thus, it is a very difficult issue to increase the use efficiency of the EUV light while achieving the intended pressure difference in a differential pumping.